From the evidence of Thucydide

The Delian league formed after the Hellenic league decided to split up. When the league started it was essentially a voluntary alliance. However, over time, Athens disregarded the original constitution and totally changed her attitude toward her allies The Hellenic league, consisting of all Greece, had fought together against Persia, with Sparta as the leader. However, after victory against the Persians, the league split into two. The Peloponnesian league, with Sparta as leaders, and the …

…contributors, like Lesbos and Chios. The same year Athens, however, made a reduction in the tribute. This was the only positive change in the treatment of the allies. It was a small change and was not much comfort for the allies. In the year 440bc there were only three genuine allies left, Samos, Chios and Lesbos, all contributing ships. Samos were arguing with Miletus over priene. Miletus appealed to Athens and Athens sided with Miletus.